Method and apparatus to reduce thermal stress when starting combined cycle power systems

Abstract

Apparatus and method to wet start the HRSG and combined cycle in the fastest time with minimum thermal stress. This wet start provides the earliest possible cooling steam during acceleration, reducing stress in the superheater, reheater and steam turbine. The gas turbine is started and loaded to full power in the fastest possible time without holds. A once-through HRSG filled with saturated boilerwater including the superheater generates dry steam during acceleration. Start apparatus positions the dryout zone in each superheater tube, controls surge swell, ensures uniform pressure rise, and controls the steam temperature. The superheater generates temperature controlled steam to cool the tubes while heating headers. Superheater and reheater tubes and headers start at saturation and increase to operating with minimum differential temperatures. The superheater evaporating boilerwater supplies constant low temperature dry steam to start the steam turbine without the use of attemporators.

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An improved combined cycle power system in which condensate from a steam turbine is heated by a once-through heat recovery steam generator by exhaust from a gas turbine, said heat recovery steam generator has at least a high pressure steam generator section and a reheater section, and may have an intermediate pressure steam generator section and a low pressure steam generator section, said high pressure steam generator has an economizer section directly connected into an evaporator section and a superheater section, the steam generator produces steam using a once-through all tubular circuit arrangement of identical said circuits comprised of a multiplicity of straight vertical finned tubes used as heat transfer elements that are connected in parallel, wherein boilerwater is heated, evaporated and superheated to produce high pressure steam for said steam turbine that may be at subcritical or supercritical pressures, each of said circuits has: an economizer circuit section, an evaporator circuit section and a split superheater circuit section, and condensate flow is connected to a high pressure economizer header, whereby it is equally distributed by means of a flow restrictor at the entrance of a water inlet tube located in a first row of each said economizer circuits, said economizer circuit section heats boilerwater prior to entering the evaporator circuit section, wherein steam is produced to flow into said split superheater circuit section positioned at the gas turbine exhaust gas entrance to the heat recovery steam generator, wherein steam is heated to the highest temperature in a last row of tubes of said split superheater circuit, said last row of tubes connecting superheated steam flow to a high pressure superheated steam header, one or a multiplicity of said headers collects the steam and discharges steam through a header nozzle, said nozzles conduct the total high pressure steam flow to a high pressure superheated steam manifold which is connected to said high pressure steam turbine admission valve connecting steam to a high pressure steam turbine section of the steam turbine, and exhaust steam from said high pressure steam turbine is connected by a one-way turbine exhaust valve to a cold reheater header that operates at an intermediate pressure, said reheater heats the cooler exhaust steam and connects steam to a hot reheater header, and said reheater section is a split reheater circuit arrangement of tubes interspaced within said split superheater circuit arrangement to effect reheater outlet temperature control, said reheater hot header connects controlled temperature reheated steam to an intermediate pressure steam manifold that connects steam to an intermediate steam turbine section by means of an intermediate steam turbine normal admission valve, and said intermediate steam turbine exhausts to a low pressure steam turbine that exhausts to a condenser, and said circuits of the high pressure steam generator and the reheater are fabricated from said straight vertical finned steel tubes which are connected in a continuous all tubular serpentine arrangement by a U-bend tube or jumper tube welded at each end, whereby a continuous once-through flow path from said water inlet tube of said economizer circuit section is connected directly through the evaporator circuit section to said last row of the split superheater circuit, and said straight finned heat transfer tubes are connected by the U-bend tubes to form the reheater that is connected from said cold reheater header to said hot reheater header in a continuous serpentine flow path, heated steam from said reheater is connected to said intermediate pressure manifold, in normal operation boilerwater and generated steam flow is in one direction from the economizer inlet header through each of said circuits to the last row of tubes in the high pressure superheater, flow can be reversed and controlled by a circuit drain system connected to each said water inlet tubes of the economizer circuit to facilitate starting said heat recovery steam generator and for quench protection in severe operational transients, said circuit drain system includes a drain line from each said boilerwater inlet tube in the first row of the economizer circuits, said drain line is connected by means of a one-way drain valve to a circuit drain header, the circuit drain headers are connected to a high capacity drain system with at least a circuit drain valve, or multiple said drain valves in parallel, to regulate drain flow rate to effect swell management in starting, said circuit drain system connected to a flash tank and the condenser or a feedwater tank, the circuit drain system also allows rapid draining of the heat recovery steam generator for freeze protection, corrosion prevention and maintenance, the tubes, lines, pipes and headers may be fabricated from any steel alloy approved by the ASME Boiler and Pressure Vessel Code Section I Power Boilers for the operating conditions, said circuits are individually vertically suspended in parallel by hangers and the inlet water tube of each of the circuits is hydraulically connected to the dryout zone in the same circuits said last row of superheater tubes, thereby providing a means to effect swell management in start up, and condensate is connected through a feedwater treatment system to a feedwater pump and flow rate is regulated by means of a feedwater control valve connected to said high pressure economizer header, temperature of steam generated is primarily controlled by adjusting the dryout zone position in said once-through circuits, said high pressure split superheater is of an arrangement with the highest temperature section of the superheater rows positioned at the gas turbine exhaust inlet to the heat recovery steam generator, wherein the last tube row is the highest temperature of said split superheater, and said reheater outlet rows are immediately downstream and lower temperature rows of said superheater are downstream of the reheater outlet rows, wherein said split superheater presents the initial heat exchanger tube surfaces to the exhaust gas entering said heat recovery steam generator, thereby resulting in a reduction in temperature in the exhaust gas flowing across said split reheater circuit tubes located immediately downstream to thereby prevent overheating of said reheater, in start up the superheater containing boilerwater evaporating also protects the superheater tubes from overheating, and a bypass arrangement for use in start up of the heat recovery steam generator consisting of the means for bypassing steam from the superheater past the steam turbine directly to the condenser, the bypass arrangement includes a high pressure steam turbine bypass valve to connect steam flow to the said cold headers of the reheater, and steam discharged from the reheater hot headers is connected to said intermediate pressure steam manifold which is connected to an intermediate turbine bypass valve connecting steam to a desuperheater before it is connected into said condenser, if installed, steam from an intermediate steam generator discharge valve is also connected to the cold reheat header, thereby all of the temperature controlled steam generated by the high pressure and intermediate pressure sections of said steam generator can bypass the steam turbine through said reheater to said condenser, thereby cooling said reheater from overheating with temperature controlled steam generated by said high pressure superheater during start up, and the high pressure superheated steam manifold is also connected to an intermediate pressure turbine start admission valve that connects controlled temperature high pressure steam to said intermediate pressure steam turbine section in start up, said intermediate pressure turbine start admission valve connects a portion of the high pressure superheater flow downstream of said normal intermediate steam turbine admission valve, exhaust steam flows from the intermediate turbine section directly into the low pressure steam turbine section which may also receive steam from a low pressure steam generator, said bypass systems are sized to condense all the steam generated from said heat recovery steam generator with said gas turbine at full load, and condensate from said condenser is connected through a water treatment system to a feedwater pump and then through a feedwater control valve regulating water inlet flow to said high pressure economizer headers, in start up said circuit drain control valve can be used to rapidly lower the boiler water level in said last row of tubes in the superheater circuits, whereby in synchronization with improvements herein disclosed will reduce differential thermal stress and prevent overheating of the superheater and reheater components when fast starting the combined cycle power system, the improvement comprising at least a start apparatus with means to control and manage swell water and minimize thermal stresses when starting said combined cycle power system without power holds as the gas turbine is loaded to full power at a rate that is approximately equal to a maximum rate of said gas turbine loading, said start apparatus is a simple weldment of mainly horizontal pipes connected to each said high pressure superheater header discharge nozzle, as part of said apparatus a horizontal drain line connects to said high pressure superheater nozzles, said superheater nozzles are conventionally arranged vertically to connect to said high pressure superheated steam manifold conventionally located exterior of a steam generator casing, the entire start apparatus is thereby arranged exterior of said heat recovery steam generator casing, the exterior location eliminates the need to add new problematic penetrations of the steam generator casing, and all: valves, actuators, sensors and control wiring are thereby exterior and are not exposed to the hot corrosive exhaust gases, in normal operation steam does not flow through any part of said start apparatus thereby preventing loss of the combined cycle power system efficiency caused by water separators, extra pipe length runs or flow turns, said horizontal drain pipes are connected to a horizontal drain manifold of said apparatus and a surge volume of at least 50% of the last row superheater tubes, and surge volume consists of: said high pressure nozzles, said superheater headers, top of said last row of superheater tubes not containing boilerwater in start up, and said apparatus piping, and a swell water carryover drain system is provided by means of said horizontal drain manifold which is equipped with an automatic pot drain valve system, a level sensor in said drain manifold is located above said high pressure superheater headers, whereby when integrated with: a gravity drain system, a geometric arrangement of pipes, said circuit drain system and said automatic pot drain valve system provides a means to ensure all said last row of tubes in said high pressure superheater are filled and vented with boilerwater to an optimum level below the superheater headers to provide space for evaporating swell water in said last row of superheater tubes during start up, and said apparatus has means to control the water level in said last row of tubes in the high pressure superheater circuits, thereby in synchronization with said circuit drain control valve provides a means to position the water level in said last row of superheater tubes prior to starting and during start up, steam pressure entering said start apparatus is controlled by a start apparatus pressure control valve discharging steam to the condenser, said horizontal drain manifold has the means to drain carryover swell water during start up automatically with a pot drain valve system, said start apparatus geometry, volume, valves and drain systems provide a means to manage and control transient swell water carryover and control superheater pressure without pressure and temperature instabilities, thereby preventing damaging cyclical thermal stress, and whereby a stable wet start of the heat recovery system is effected, and superheated steam is generated during the gas turbine acceleration without causing overheating damage and without life reducing high differential temperature stresses of conventional systems that require problematic inter-stage and terminal attemperators for said superheater and reheater, and said start apparatus has a small bypass flow control valve, said bypass flow control valve controls flow used in starting hot said heat recovery steam generators requiring start up shortly after a shutdown, said small bypass valve meters saturated steam flow through said high pressure headers to cool the superheater headers and prevent quench damage in hot start up, and the start apparatus has a nitrogen supply connection from a nitrogen blanketing system, or other nitrogen source, to pressurize said start apparatus and said heat recovery steam generator when said steam generator cools and steam pressure reduces to near ambient air pressure, whereby the nitrogen gas provides a means to allow gas-pressurized-forced draining of said circuits required for fast starts when said heat recovery steam generator is cold. 
     
     
         9 . A method for starting said improved combined cycle power system in accordance with  claim 8  wherein said combined cycle is started from a warm condition, typically after an overnight shutdown, with said heat recovery steam generator containing boilerwater at high saturated steam pressure, boilerwater is primarily in the evaporator section and the economizer section, the start up method is to fill all said circuits with boilerwater to an optimum level below said high pressure superheater headers, and the gas turbine is started to full load in the shortest time without holds, the steam generator is concurrently started wet and rapidly generates dry steam during acceleration, and the gas turbine exhaust gas entering said heat recovery steam generator rapidly heats said superheater last row tubes from saturation temperature to superheated steam that is controlled at a low temperature allowable to start and load the steam turbine, thereby cooling said superheater and said reheater while metal components evenly ramp-up from saturated steam temperature as steam flow is regulated by means of a bypass system past the said steam turbine to the condenser, thereby minimizing thermal stresses in the steam generator without the use of problematic attemperators, and loading said steam turbine to full power can be accomplished in minimum time while differential expansion clearances and thrust loads are controlled, the starting method comprising the steps of:
 a) prior to the gas turbine start, feedwater flow is regulated by said feedwater control valve and pumped into said high pressure economizer headers to displace boilerwater and steam in said economizer and evaporator sections, and conducting boilerwater water to flow through said high pressure superheater header discharge nozzles and through said start apparatus horizontal drain manifold, and water flow continuing vertically upwards in the manifold to a level sensor above said high pressure superheater headers, thereby ensuring all headers and each said last row of tubes in the high pressure superheater are vented and full of saturated boilerwater by means of gravity back-flow into each said header and tubes from the relatively high water level above each header facilitated by the geometric pipe arrangement allowing venting and back-flow; and 
 b) immediately preceding said gas turbine start, the water level from said horizontal drain manifold is lowered to identical levels in each said high pressure header discharge nozzle by draining said horizontal drain manifold by means of opening said pot drain valves, whereby the water level drains to the bottom of said horizontal drain pipe connecting the nozzles to said horizontal drain manifold, and thereby positioning the water level in said header nozzles several feet above each said high pressure superheater header as a result of the geometric arrangement of pipes, thereby providing additional swell surge volume and placing said start apparatus in a ready-to-start status; 
 c) starting said gas turbine and loading the gas turbine to full power at approximately equal to a maximum rate of said gas turbine; 
 d) at the gas turbine flame detection signal, a small specific volume of water is drained from the volume of: said high pressure superheater header discharge nozzle, said header and said top of last row of tubes in said high pressure superheater circuits by means of opening the circuit drain control valve, and the circuit drain valve is maintained open for a specific time period, wherein the time period for opening is a function of boilerwater saturation pressure and percent opening of the flow calibrated drain valve, and thereby removing a specific volume derived from each said circuit, thereby lowering the water level to a specific position near the top of each said last row of tubes in the high pressure superheater, and whereby the level is controlled by a computer predicted drain flow rate from a predictive feedforward algorithm based on measured gas temperature, gas turbine speed and gas turbine characteristics during gas turbine acceleration and loading, and as gas temperature and flow rapidly increase, steam is rapidly generated, earlier than conventional steam generators, early generated steam prevents overheating of said last row superheater tubes as the gas turbine accelerates and is loaded, steam pressure and temperature are controlled by means of modulating open the start apparatus pressure control valve, thereby allowing steam and some swell water to flow into said start apparatus space volume, and concurrently said high pressure circuit drain control valve is modulated opened, and thereby creating space for swell water near the top of each said last row of high pressure superheater tubes as boilerwater is rapidly drained from said circuit drain header to a flash tank, and the rapidly increasing pressure contracts the volume of swell water, whereby the majority of the swell water remains in the increasing space above the water level created in said last row of tubes, and at the same time steam flow is conducted through said start apparatus by modulating open said start apparatus pressure control valve, this steam flow passing through the last row of the high pressure superheater is cooling said last row of the superheater tubes, preventing overheating damage while simultaneously heating said superheater headers from saturation temperature to superheat temperature and thereby minimizing differential temperatures and stress at header-to-tube joints, and concurrently any carryover swell water from said high pressure superheater is drained by means of the start apparatus's horizontal drain manifold through said automatic pot drain valves opening as water accumulates, the last row of superheater tubes containing evaporating boilerwater greatly reduces the exhaust gas temperature upstream of said reheater, thereby preventing overheating damage to the reheater tubes, said feedwater control valve is manipulated to control the position of the dryout zone in said superheater to regulate temperature of the superheated steam to achieve dry steam as soon as possible, and said start apparatus pressure control valve is manipulated, regulating pressure, and when dry steam is sensed in the high pressure superheater discharge, pressure control is transferred by closing said start apparatus pressure control valve synchronized with opening said steam turbine bypass system to the condenser and thereby conducting steam flow from said high pressure superheater past said steam turbine through the reheater, and concurrently said intermediate pressure steam generator superheater section discharge valve is opened conducting steam through the reheater, steam from said hot reheater discharge header flows into said intermediate pressure manifold conducting it through said intermediate steam turbine bypass valve to the condenser, both the high pressure steam turbine and said intermediate turbine bypass valves are manipulated, thereby controlling both the high and intermediate steam pressures by conducting steam to an alternative path to said condenser, whereby steam flowing through said reheater cools said reheater tubes preventing overheating damage and heats said reheater headers, thereby reducing damaging thermal stress at reheater header to tube joints; and 
 e) the discharge steam temperature from said high pressure superheater is increased from saturated boilerwater temperature by means of modulating said high pressure feedwater control valve to a lower flow rate, thereby decreasing water flow into said high pressure circuits and thereby positioning the dryout zone further away from said high pressure steam superheater header, and thereby increasing the area of said superheater and increasing outlet steam temperature which is controlled at a constant allowable low temperature level to permit starting and loading the steam turbine, and by means of opening and modulating said high pressure steam turbine admission valve and said intermediate steam turbine start admission valve, the steam flow to both turbine sections is divided to flow at a rate to each turbine section to conform to allowable criteria for a specific steam turbine section's thermal stresses, rotor clearances, and thrust, as said bypass system controls pressures by means of an alternative steam flow path until substantially all of the steam generated by said high pressure superheater is split between said high pressure steam turbine section and said intermediate steam turbine section, the high pressure steam turbine exhaust is conducted through the reheater and then through said intermediate steam turbine bypass valve to the condenser, and the total flow from the high pressure steam generator to said high pressure and intermediate pressure turbine sections at the low allowable turbine starting temperature is greater than rated flow at the much higher operating temperature and pressure, and wherein the flow is divided to optimize rapid warmup, each turbine section receives about 55% to 65% flow compared to the normal full rated flow at 100% power, and the high pressure steam turbine exhaust flows through a one-way valve through said reheater cold header and the steam temperature is increased in the intermediate steam manifold but flows past the intermediate steam turbine through the bypass system at a flow rate of about 55% to 65% and discharged to the condenser until the intermediate steam temperature exiting the reheater is increased, steam exhaust from the intermediate steam turbine is conducted to a low pressure steam turbine that exhausts directly to said condenser; and 
 f) the temperature of the steam flow from the split high pressure superheater is increased slightly by decreasing the high pressure feedwater flow by means of modulating said feedwater control valve, by this means the location of the dryout zone row in the superheater relocates row by row downstream from an outlet row of the split reheater thereby causing a step increase in gas temperature entering said outlet row of the split reheater, as a result the split reheater outlet steam temperature ramp-rise rate accelerates from said reheater hot header resulting from the high pressure superheated steam temperatures smaller increase, and thereby increasing the temperature of the intermediate pressure steam temperature to an allowable temperature to be admitted to the intermediate steam turbine with a relatively smaller increase in the high pressure steam temperature, and the normal intermediate steam turbine admission valve is modulated open to continued steam flow loading of the remaining flow of about 45% to obtain 100% flow through the intermediate steam turbine, and in synchronization with modulating closed said intermediate steam turbine start admission valve the high pressure turbine admission valve is modulated further open to divert the additional high pressure steam flow made available, and thereby increasing the high pressure steam flow by approximately 45%, and said steam turbine bypass system controls high pressure and intermediate pressure with an alternative path to the condenser modulates reducing flow past the intermediate steam turbine section, and the additional flow is admitted through the high pressure steam turbine section compatible with allowable steam turbine stress and differential expansion until of 100% of the steam flow is admitted to the high pressure steam turbine section and the intermediate steam turbine section at a low allowable start temperature; and 
 g) after substantially all the steam generated by the heat recovery steam generator, except the low pressure steam generator, is admitted into said high pressure steam turbine section and intermediate pressure steam turbine section at the low allowable turbine start temperature, the steam temperature from said high pressure superheater is increased by modulating said feedwater control valve and said bypass systems controlling the heat recovery steam pressure by means of an alternative steam flow path, and said intermediate steam temperature increasing as the high pressure turbine exhaust temperature increases and the dryout zone is relocated towards the full load design conditions, at a rate compatible with allowable steam turbine stress and differential expansion until rated operating temperature and pressure at full power is obtained for said high pressure steam turbine section and said intermediate steam turbine section; and 
 h) applying conventional methods, a low pressure steam generator control system for said low pressure superheater section of said heat recovery steam generator concurrently supplies the additional steam flow to said steam turbine low pressure section to achieve full power and efficiency from said steam turbine. 
 
     
     
         10 . The method of  claim 9  wherein the heat recovery steam generator is cold, typically after a weekend or longer shutdown of combined cycle power system, and other operations or maintenance, when the boilerwater steam pressure approaches ambient air pressure, prior to the start up method of pumping feedwater into said economizer header to displace boilerwater, said start apparatus nitrogen supply connection is opened and nitrogen is introduced to replace saturated steam as it condenses, the nitrogen is to prevent air ingress and corrosion of said heat recovery steam generator, and sufficient nitrogen is added to maintain adequate motive pressure after it is compressed by boilerwater pumped into said high pressure steam generator circuits, thereby the compressed nitrogen becomes the motivate pressure for draining said high pressure steam generator circuits in start up until sufficient steam is generated to replace the nitrogen as it is discharged to the condenser. 
     
     
         11 . The method of  claim 9  wherein said heat recovery steam generator is hot and the high pressure superheater headers are relatively hot due to shutting down at the high operating temperature, and prior to start up said high pressure superheater header temperature is reduced from the high superheat temperatures to a less damaging warm start temperature close to saturated boilerwater temperature, thereby preventing high thermal quench stresses during filling said high pressure superheater with boilerwater at the much lower saturated steam temperature, if said combined cycle power system is placed on impending dispatch service, immediately after the gas turbine shut down, said small bypass flow control valve in the start apparatus is opened, conducting saturated steam flow past the start apparatus pressure control valve and thereby through the high pressure superheater headers, cooling them at a flow rate limited to prevent carryover of boilerwater from said heat recovery steam generator, and from the start apparatus steam is discharged to said condenser, the steam discharge temperature from said high pressure header is measured by a temperature sensor in one or more of said horizontal drain pipes connected to the high pressure header nozzle of said heat recovery steam generator, as saturated steam flow continues cooling the high pressure superheater headers the steam temperature in the in the horizontal drain pipe is reduced and is an indication of when said high pressure superheater header is ready for a warm start, said small bypass control valve is closed to conserve thermal energy when this temperature is obtained, and said combined cycle power system is in a ready-to-start status. 
     
     
         12 . A start apparatus in accordance with  claim 8  that has important performance and operational features comprising:
 a) no increased pressure loss or cycle efficiency reduction caused by said start apparatus when installed in normal combined cycle power system operation; 
 b) all start up sensors, instrumentation, actuators, valves and wiring are installed external of the steam generator casing and thereby removing them from effects of hot corrosive exhaust gas, thereby enhancing: availability, reliability and maintainability; 
 c) no additional problematic steam pipe penetrations of the steam generator casing; 
 d) elimination of problematic inter-stage and terminal superheater and reheater attemperators used in conventional heat recovery steam generators; 
 e) lower thermal stress in fast start heat recovery steam generators for combined cycle power systems resulting in shorter start time, longer cyclic life, higher availability and lower operating costs.